A physicochemical process for refining metal surfaces is described and claimed in Michaud et al U.S. Pat. No. 4,491,500, which process involves the development, physical removal and continuous repair of a relatively soft coating on the surface. High points are leveled through mechanical action, preferably developed in vibratory mass finishing apparatus, and very smooth and refined surfaces are ultimately produced in relatively brief periods of time.
The patentees teach that the process can be carried out using either a part-on-part technique or by incorporating an abrasive mass finishing media; e.g., quartz, granite, aluminum oxides, iron oxides, and silicon carbide, which may be held within a matrix of porcelain, plastic, or the like. As described therein, the effectiveness of the process is evidently attributable to the selective removal of surface irregularities, which removal has been facilitated by chemical conversion of the metal to a softer form.
Although the Michaud et al process is most effective and satisfactory, it is self-evident that the realization of even higher production rates and improved quality of the ultimate workpiece surface would constitute valuable advances in the art. This would of course be especially so, moreover, if those benefits were achieved by a process that is more economical, facile and environmentally attractive to carry out.
To achieve ultimate refinement of the metal surface, it will generally be desirable to finish the Michaud et al process with a burnishing step, which may be carried out by treatment of the parts in a mass finishing unit charged with a so-called burnishing media and an aqueous alkaline soap solution, the latter being inert to the metal. Such burnishing media will typically be composed of mineral oxide grains fused to a hard, dense, non-abrasive cohesive mass; it is also commonly known to use steel balls for burnishing metal parts.
It has in the past been standard practice to first treat the workpieces in a vibratory bowl containing abrasive media (e.g., grit-filled ceramic loaded to about 20 to 40 percent with the abrasive grains, when the operation is chemically promoted), and to then transfer them to a second bowl filled with a burnishing media; however, doing so is obviously inconvenient, time-consuming, and expensive. The process described by Michaud et al can be employed to produce burnished parts, without transferring them to a second bowl, by using a relatively nonaggressive cutting medium (e.g., a ceramic containing 10 to 15 percent of abrasive grit). In such a procedure, the initial, surface-refinement phase is carried out with a reactive solution which produces the conversion coating on the parts, followed by a flushing step and then a flow of a burnishing soap solution, with the equipment in operation.
Although highly advantageous, such a method may not produce ultimate refinement of the metal surfaces (e.g., specular brightness), since it is characteristic of abrasive media that they scratch the metal surfaces. Also, to be effective the grit particles of such media must continuously fracture, providing fresh, sharp edges to achieve the cutting function; it is obvious that, for environmental reasons, the solutions used in the process must therefore be treated to remove the particulates so produced, as well as to remove the powdery residue and grains released by attrition of the ceramic matrix.
Accordingly, it is the broad object of the present invention to provide a novel and highly effective process for the refinement of metal surfaces utilizing a physicochemical finishing technique.
It is a more specific object of the invention to provide such a process by which enhanced surface refinement may be achieved at a faster rate than has heretofore been realized by comparable means.
It is a further object of the invention to provide a process having the foregoing features and advantages, which is also more economical and facile to carry out than earlier processes of the same kind, and which offers environmental advantages.
It is another specific object to provide a novel physicochemical process by which relatively rough metal surfaces can be brought to a specular condition in one step; i.e., with one media and without transfer of the parts.